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XXIX.— The Visual, Grating and Glass-lens, Solar Spectrum (in 1884). 

 By C. Piazzi Smyth, F.R.S.E., and Astronomer-Royal for Scotland. 

 (Plates LXXXIII.-CXLIII.) 



CONTENTS. 



PAGES 



519-524 

 524-526 

 526-528 

 529-531 

 531-532 



Part I. The Observations ; their objects, and the mode of compassing them, 



Meteorological Appendix to the Same, .... 



Part II. Reduction of the Instrumental, to a Wave-Number, Scale, 



Part III. Graphical representation, in place of printed numbers alone, '• . 



Part IV. Indexing by Colour, ...... 



Part V. Variations of Temperature and other probable sources of Minute Disturbance, . 532-537 



Part VI. Appearances and Disappearances of Terrestrial Water-Gas lines in the Solar Spectrum, 537-539 



Part VII. Some of the results arrived at touching true Solar lines in the year 1884, . 539-541 



Part VIII. Testimony of Successive Gaseous Groups as to the most practical of Natural 



Spectrum Scales, ......... 541-543 



Part IX. The Map in 60 Plates and with an Index Plate, LXXXIII.-CXLIII. 



Part I. — The Observations ; their objects, and the mode of 



COMPASSING THEM. 



Throughout the year 1884 the sun was seen over most countries under 

 peculiar atmospheric circumstances ; and these, instead of being confined to 

 low altitudes, were never more conspicuous than during the summer of the 

 year, and noon-tide of each day, and in the clear air on mountain heights of 

 every country, whenever the sky was more or less free of actual clouds. 



On such occasions then, the usual phenomenon to be noticed by the eye, 

 was, that nothing like blue sky could be witnessed near the sun. But in 

 place of that, there was a broad glare of whitish light extending for several 

 degrees around the luminary ; and beyond that range, or over 20 degrees 

 distant from him, there spread a wide reddish haze, passing into purple, and at 

 greater distances into blue, but nowhere a very decided and deep blue sky. 



That the medium producing this appearance was terrestrial rather than 

 Solar, was indicated by similar effects being seen about the full moon. But 

 that its locality must have been far higher than the ordinary clouds was still 

 more conspicuously proved by the said clouds always appearing in front of, or 

 backed by, the peculiar reddish glare ; and then showing their own cloud- 

 tints, greenish-white on their illuminated edges, and blue-gray on their shaded 

 sides, in most pronounced chromatic relief. 



The upper air, to produce any such effect, must at the very least have 

 been filled with far more than the average amount of those minute dust 

 particles, which are always floating about there in greater or less quantity far 

 above the level of all ordinary massive clouds of watery vapour. And though 

 some of the extra amount of this higher dust may have been derived from 

 several other sources, yet the general opinion of most men of science in 



VOL. XXXII. PART III. 4 R 



